Prefix category methods

This PR prefixes XMPPFramework's categories on `NSData`, `NSNumber`, and `NSXMLElement`. For backward compatibility, the non-prefixed originals remain, but are marked as deprecated with a message indicating the new name. The deprecated categories can be excluded by defining `XMPP_EXCLUDE_DEPRECATED`.

For details on why category methods should be prefixed, see [Customizing Existing Classes](http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html#//apple_ref/doc/uid/TP40011210-CH6-SW2).